High frequency choke coil



Oct. 18, 1960 H. D. SHAMBLIN HIGH- FREQUENCY CHOKE COIL Origina .l Filed Nov. 25, 1953 LA LB CA FPR r Fsn RA Re FIGBA.

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INVENTOR. HENEGAR D. SHAMBLIN United States Patent 2,957,124 HIGH FREQUENCY CHOKE COH,

Henegar D. Shamblin, Miami, Fla., assignor to Aeronautical Communications Equipment, Inc., Miami, Fla., a corporation of Florida Continuation of abandoned application Ser. No. 394,414, Nov. 25, 1953. This application Nov. 26, 1956, Ser. No. 624,350

2 Claims. (Cl. 323-82) This invention is a continuation of SN. 394,414, filed November 25, 1953, now abandoned, and relates to high frequency choke coils, and has particular reference to methods and apparatus for providing chokes that are useful over a wide range of frequencies.

An element possessing the characteristics of a low D.-C. resistance and a high A.-C. impedance over a certain range of frequencies is useful in many applications and accounts for the importance of the high frequency choke coil, a device embodying the above-recited characteristics. At radio frequencies, it is well known that such chokes will exhibit parallel resonance effects due to the distributed capacity of the coil turns. Accordingly, in most circuit applications, a choke is employed that will be operating near its parallel resonant frequency in order to gain the advantages of the high impedance presented at that point.

It is apparent that the lower usable frequency of a choke coil disposed, for example, in the plate circuit of a power amplifier, is determined by its inductance. As the frequency increases, the choke will pass through paralled resonance and will exhibit series resonance effects attributable to the distributed capacity effectively in series with the coil inductance. This condition reduces the impedance of the choke drastically and results in a current flow therethrough sufi'lcient in most instances to overheat and destroy the coil and, accordingly, the upper frequency limit of the choke must be somewhat below this point.

Efforts have been made to obviate these difiiculties caused by the series resonance effects by reducing the inductance of the choke and thereby moving the series resonance point to a higher frequency. However, in circuits wherein a wide range of frequencies are encountered, lowering the inductance will undesirably prevent the employment of lower frequencies, as pointed out above.

In accordance with the present invention, the series resonance problem is overcome without raising the lower limit of the frequency that may be utilized. In accomplishing this, a magnetic core of predetermined dimensions is inserted in one end of the choke which, on first consideration, would appear to lower the series resonance point by raising the coil inductance. However, it has been found that although the inductance is revised somewhat upwardly, the series resonance point is not lowered but is actually raised a substantial. amount by the retuning effect of such core on the high R.-F. side of the coil.

These and further advantages of the invention will be more readily understood when the following description is read in connection with the accompanying drawings in which:

- Figure 1 is a view in elevation of a high frequency choke coil constructed in accordance with the present invention;

Figure 2 is a schematic diagram of one form of circuit that may utilize the choke illustrated in Fig. l; and

Figures 3A, 3B and 3C are schematic diagrams helpful in describing the invention.

Referring to the drawings and more particularly to Fig. l, a high frequency choke coil 10 consists of a coil form 11, preferably tubular in form, having a coil 12 wound thereon and provided with terminals 13 and 14. It should be noted that the terminal 13 is adapted to be joined to the high R.-F. potential in any circuit in which the choke 10 is used.

Since the choke 10 is to be employed at high frequencies, it is preferable to construct the coil form 11 from ceramic to minimize losses. Furthermore, the coil 12 is preferably tightly wound with each turn lying against the preceding turn in a solenoid type winding. However, other coil arrangements that do not introduce an excessive amount of distributed capacity into the choke may be desirable in some instances. Furthermore, the use of a self-supporting coil without a coil form may be advantageous in certain applications.

Adapted to be placed in the choke 10 is a magnetic core 15, shown both in elevation and in position in the choke It) in Fig. 1, and in this instance cylindrical in form to fit snugly in the coil form 11. The core 15 is preferably provided with a hollowed out portion 16 to aid in cooling and to economize on materials. It will be understood that a solid core may be employed, the only limitation being that it must be magnetic. It is also preferable to construct the core 15 from powdered material in order to minimize eddy current losses.

The length of the core 15 with respect to the coil 12 will vary somewhat according to the type of coil and the ratio of coil length to coil diameter. It has been found that for optimum operation of chokes wherein the coil length is substantially greater than the coil diameter, the core 15 is preferably between one fifth and one half as long as the coil 12. Furthermore, it has been discovered that for the best performance of the high frequency choke 10, the core 15 must be disposed therein at the high R.-F. side of the choke 10 defined by terminal 13 in Fig. 1, and substantially coinciding with the end of the coil 12. It will be understood that the core 15 may be adjusted a small distance inwardly or outwardly with respect to the coil end in order to obtain optimum results.

To aid in a better understanding of a choke constructed in accordance with the present invention, reference will be made to Fig. 2 wherein a power amplifier 20 supplying a load 22 is illustrated. The amplifier 252*, shown herein as a triode with the conventional components in circuit therewith, is driven from a signal applied to the input terminals 21. The output signals are generated across the choke 10, provided with the terminals 13 and 14 constructed as described above, and coupled to the load 22 through a coupling capacitor 23 and a conventional impedance matching network 24. A condenser 25 is connected between the 13-!- end of the choke 10 and ground to bypass any R.-F. energy found at that point.

In the operation of the circuit illustrated in Fig. 2, it will be assumed that signals ranging from about 1.6 megacycles per second to 22 megacycles per second are coupled to the input terminals 21. An ordinary air core choke coil having the inductance necessary to present an impedance at the lower frequencies sufiicient to afford an R.-F. current and related 1 R losses in the choke of such a magnitude that the heating effects will not be objectionable, will be subject to series resonance effects substantially below the upper frequency limit and will, accordingly, be unsuitable. However, with the choke 10 constnlcted in accordance with the present invention sufii cient inductance may be provided for low frequency operation since the insertion of the magnetic core 15 having predetermined dimensions in the high R.-F. side of the choke 10, as explained heretofore, will substantially raise the frequency at which series resonance elfects are ex- 3 hibited. It should also be noted that due to the increase in overall inductance of the coil afforded by the core, the usable lower frequency will be somewhat extended.

One theory which accounts for the aforementioned unusual and unexpected results will be discussed with particular reference being made to Figs. 3A, 3B and 3C. In Fig. 3A, a schematic diagram of an equivalent circuit of the choke 10 at its parallel resonant frequency is illustrated. A voltage F represents an applied signal at the frequency necessary to obtain parallel resonance between the inductance L of the coil and the effective parallel capacitance C of the coil, the resistance R also being shown. At the frequency P accordingly, X will equal X and an extremely high impedance will be presented by the choke 10.

However, as the frequency of the signal is increased to a frequency F the choke =10 exhibits series resonance effects and the equivalent circuit of the choke 10 will be primarily of the form illustrated schematically in Fig. 3B before insertion of the core 15. The inductance of the coil is represented by L the resistance by R and the effective series capacitance by C It is evident that at the frequency F the capacity C which is substantially smaller than C in Fig. 3A, becomes important so that X is equal to X providing a low impedance having a value of substantially R In order to eliminate the series resonance effects as the signal reaches the frequency F the core 15 is introduced into the choke in the manner described heretofore and the resulting equivalent electrical circuit which explains the behavior of the choke 10 at the frequency F is shown in Fig. 3C. The core 15 retunes the high R.-F. end of the choke to provide a parallel combination of an inductance L and a resistance R in one arm and a capacitance C in the other arm of the circuit. This network is in series with an inductance L a resistance R and a capacitance C With the core properly positioned, the impedance of the choke at the frequency F is extremely high so that the useful range of the choke is extended well above this frequency.

In measuring the terminal impedance of the choke at the frequency F with a Q meter after the insertion of the core 15, the above-mentioned high impedance was found. Simultaneously, a grid dip meter was coupled into the choke and a resonant condition was discovered, indicating that the insertion of the core 15 changed the condition of at least the section of the coil 12 adjacent the core 15 from series to parallel resonance. It is evident that it will be immaterial whether the combination of L and C are series resonant since the extremely high impedance of the parallel network will be the controlling factor.

An example of a high frequency choke constructed in accordance with the invention will be presented in order to demonstrate the advantageous results flowing therefrom. The coil may comprise a ceramic coil form 2%" in length and having a inner diameter and a 1" outer diameter wound with 110 /2 turns of #27 enamel coated copper wire forming a coil 2" in length terminated by studs disposed and 7 inwardly from the high and low ends of the coil form, respectively. The core may be a conventional tubular powdered iron core having a A inner diameter, a outer diameter and a length of /2". The core is preferably positioned inwardly 7 from the high end of the form if the choke is operated in free space relatively remote from metal surfaces. If an L-shaped aluminum shield thick and 2" wide has its short leg fastened to the high end of the form so that the long leg is spaced 1 7 from the coil axis, the core is preferably spaced from the high end of the form. In either case, the core is secured in position by means of an adhesive such as an electrical varnish designed for R.-F. applications.

The operating characteristics of the above exemplary high frequency shielded choke coil are, as will be seen from the following details, very advantageous. Without the magnetic core, the parallel resonant frequency of the choke was about ll megacycles (mc.) and the lowest series resonance frequency about 20.7 me. A second series resonance frequency was also discovered at about 29.5 mc. -With the core disposed in the choke so that its outside end was substantially in coincidence with the outside end of the coil, the choke characteristics were greatly modified. The parallel resonant frequency remained at about 11 me. but the series resonant point formerly found at about 20 me. was completely eliminated and at this point, the impedance of the choke was extremely high. This change, of course, permits the use of the choke at substantially higher frequencies, the limit in this instance approaching the next series resonance point which appeared at about 28.3 mc., not far removed from that indicated in the choke before the insertion of the core.

A specific embodiment of this invention has been described to present quantitatively the distinct advantages provided over other high frequency choke coils. It will be understood that the invention is not to be limited in any manner by the exarnple presented nor by any of the above-described embodiments of the invention which are illustrative only and modifications thereof will occur to those skilled in the art. For example, coils of different form may be employed as for example, coils having spaced turns, double layer coils, basket weave coils, etc., and the magnetic core may be suitably positioned therein in accordance with the invention. Therefore, the invention is not to be limited to the specific apparatus dis closed herein but is to be defined by the appended claims.

I claim:

1. Electric circuits comprising a radio frequency choke coil exhibiting series resonance effects at a frequency determined by its inductance and distributed capacitance, means energizing the core with radio frequency energy to provide a high radio frequency coil end, a magnetic core less than one-half the length of said coil, and means fixedly positioning said core in the high radio frequency coil end, the size of the magnetic core and its fixed position in the coil end being determined by the electrical characteristics of the coil, the core being operative to cause parallel resonance effects in the high radio frequency coil end that counteract the series resonance effects and extend the useful operating range of the coil.

2. Electric circuits comprising a radio frequency choke coil in the form of a solenoid exhibiting series resonance effects at a frequency determined by its inductance and distributed capacitance, means energizing said solenoids with radio frequency energy to provide a high radio frequency solenoid end, a magnetic core less than one-half and greater than one-fifth the length of said solenoid and closely fitted therein, and means fixedly positioning said core in the high radio frequency solenoid end with its outside end substantially coinciding with the outer edge of said solenoid, the size of the magnetic core and its fixed position in the solenoid end being determined by the electrical characteristics of the solenoid, the core being operative to cause parallel resonance effects in the high radio frequency solenoid end that counteract the series resonance effects and extend the useful operating range of the coil.

References Cited in the file of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N09 2,95%124 October 18 1960 Henegar D6 Shamblin It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4 line 5O for solenoids" read me solenoid Signed and sealed this 11th day of April 1961o (SEAL) ERNEST iYY ARTHUR w. CROCKER Actmg Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No; 2,95%124 October 13, I960 Henegar De Shamblin It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column-4 line 5O for "solenoid-S"- read solenoid Signed and sealed this 11th day of April 1961o (SEAL) Afloat:

E N T W';.,,. LYYF ARTHUR w. CROCKER fiifiiber Actmg Commissioner of Patents 

